The logo of the game made it pretty clear who's going to get top billing. Squall and Rinoa were identified right from the start to be the main characters with the rest of them being the supporting cast. I think it plays out better that way.

It's not like each of the characters don't get their little moments anyway. They do delve into each of the other characters at various points of the story:
Selphie - Missile Base, Trabia
Irvine - trying to shoot Edea, basketball court at Trabia
Zell - smurfing up at Timber TV Station, assuming command after Squall goes to space
Quistis - Training Centre and... okay, maybe not all the characters.

Cait Sith and Amarant were pretty much dead weight story-wise in VII and IX anyway. I don't think VIII should be singled out here for this.